Is transporting passengers carrying alcohol by a taxi driver considered a sin, and is he permitted to do so if the situation recurs?

If you know that they are carrying alcohol, it is not permissible for you to give them a ride in your car, as this would be assisting in sin, for the Prophet, peace and blessings be upon him, said: "Cursed is the carrier of alcohol and the one for whom it is carried."

As for the fare you received, you may benefit from it because it was in exchange for transporting people, not for carrying alcohol.